The influence of aging of four cosmetic formulations on the mechanical properties of plastic packaging is studied. Four thermoplastic materials in the form of bottles were used: polyethylene, polypropylene, polyethylene terephthalate, and polyethylene terephthalate glycol copolymer. Bottles were filled with cosmetic formulations and put into a steam room at 42°C for 15 days, 1 month, 2 months, and 3 months. Periodically, bottles were emptied, and mechanical properties and performances (tensile test, compression, and under-pressure tests) were studied as well as the wall thickness, the glass transition temperature (Tg), the crystallinity, and the kinetic sorption by Fourier transform infrared spectrometry. A significant change of mechanical properties was observed after the sorption of cosmetic components into the materials: this change depending on the type of material (especially for polyethylene and polypropylene) and on the cosmetic formulation. These changes must be minimized, particularly in the light of reducing packaging weight. This work leads to a new optimization methodology permitting the anticipation of packaging design.